1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and its manufacturing method.
2. Description of the Prior Art
Conventional solid electrolytic capacitors are manufactured in the subsequently-described manner.
First, an oxide film is formed through anodic oxidation on the surface of a sintered body made of a valve metal such as tantalum (Ta), aluminum (Al) and niobium (Nb). Then, after a manganese nitrate water solution is impregnated into it, a manganese dioxide (MnO.sub.2) layer which serves as an electrolyte is formed on the oxide film through thermal decomposition of the manganese nitrate. Lastly, a graphite layer and a silver layer for forming an electrode are formed on the manganese dioxide layer.
Subsequently, an example of a solid electrolytic capacitor which is manufactured in the above-mentioned manner will be described.
FIG. 1 shows a cross-sectional structure of a solid electrolytic capacitor made of tantalum. FIGS. 2A to 2C are cross-sectional views showing a manner in which a tantalum pentoxide layer (Ta.sub.2 O.sub.5 layer) 11 and a first manganese dioxide layer (first MnO.sub.2 layer) 12 are formed on a part of a porous tantalum sintered body (Ta sintered body) 10 of the solid electrolytic capacitor of FIG. 1. A sintered body layer 15 shown in FIG. 1 is formed by forming the tantalum pentoxide layer 11 and the first manganese dioxide layer 12 on the surface of the tantalum sintered body 10 as shown in FIGS. 2A to 2C. A wire 5 made of tantalum and constituting a terminal is electrically connected and fixed to the tantalum sintered body 10 (Fig. 2A). At the time of anodic oxidation of the tantalum sintered body 10, the tantalum pentoxide layer 11 is also formed on the wire 5 (FIG. 1).
As shown in FIG. 1, a second manganese dioxide layer (second MnO.sub.2 layer) 20 is formed on the surface of the sintered body layer 15. The layer 20 covers the external surface of the sintered body 15 with hardly intruding into the sintered body layer 15. A graphite layer (Gr layer) 30 and a silver layer (Ag layer) 40 are formed on the second manganese layer 20.
The above-described conventional capacitor offers the following problems.
Firstly, the conventional capacitor is weak to external stress since the sintered body made of a valve metal has a poor mechanical strength. To lighten the stress, the second manganese dioxide layer is normally formed to be thick (30 to 40 .mu.m). However, since the oxide film is extremely thin (approximately tens to thousands of angstroms), it is weak to physical stress, and is apt to break down.
Secondly, the conventional capacitor is high in equivalent series resistance since manganese dioxide which serves as an electrolyte is high in resistivity (approximately 1 to 10.OMEGA..cm).
Thirdly, it is difficult to improve the damp resisting property of the capacitor since the manganese dioxide layer has a poor water resisting property.